This invention relates to fluid-pressure operated boosters for vehicle braking systems.
Known boosters for vehicle braking systems include a movable wall which applies a force to an output member, and the force exerted by the movable wall is augmented by fluid-pressure applied to the wall under the control of a mechanical valve operated by a pedal to energize the booster.
When the booster is pneumatically operated, chambers an opposite sides of the wall are normally subjected to equal fluid pressures, for example vacuum, through the open valve. Operation of the pedal first closes the valve to isolate the chambers from each other, and then operates the valve to admit fluid at a different pressure, suitably atmospheric air into one of the chambers, whereby to energize the booster by subjecting the movable wall to a differential pressure which augments the brake-applying force from the pedal.
When the booster is hydraulically-operated operation of the valve, suitably a spool valve, by the pedal causes hydraulic fluid under pressure, suitably from an hydraulic accumulator, to be admitted into a boost chamber, whereby to act on a boost piston, which comprises the movable wall, and energize the booster by pressurising the boost chamber.
It is known from EP-A-0 267 018 to control the operation of such known boosters independently of the pedal by the use of solenoid-operated valve means which are responsive to signals sensed by wheel speed sensors. This enables the brakes to be applied independently of the pedal to achieve traction control of a vehicle by applying the brake on a spinning wheel.